I. Field of the Invention
The invention relates to Fluidized Catalytic Cracking (FCC).
II. Description of the Prior Art
Two significant, and somewhat related, problems that exist today in many FCC units are particulates emissions and fluidization. Particulate emissions are to a great extent a legislatively created problem. FCC's on average emit fewer particulates today than they did even a decade ago. Particulate emissions occur because the FCC process inherently produces catalyst fines, submicron and less than 5 micron sized particles which cannot be effectively captured even by modem high efficiency cyclones. Some of the fines end up in various products, typically as the slurry in the FCC slurry oil. Particulates there cause scouring in diesel engines which burn this oil as fuel, and cause quality control problems in plants which convert slurry oil into carbon black. At least no laws are broken if heavy liquid products have trace amounts of micron and submicron sized particulates.
Most fines produced in an FCC exit the unit with the flue gas from the regenerator. Many units use third stage separators, typically hundreds of small diameter cyclones, to capture the extremely fine particulates. Some FCC units employ wet scrubbers, electrostatic precipitators, or even baghouses to reduce particulates emissions. Thus modem FCC units are pretty good at removing fines produced by the FCC unit.
Much of the reduction of particulates emissions is due to the talents of people making the catalyst. Catalyst manufacturers have increased the durability of the catalyst used in the FCC process over the last fifty years, significantly reducing the amount of fines inherent from the use of such material. These advances, however, have not always been sufficient to meet ever more stringent limits set by law in many areas on particulates emissions. In further responding to this problem, many FCC catalyst manufacturers have increased the size of the FCC catalyst. The theory is that larger size particles are easier to retain in the system. The size of FCC catalyst, both as supplied by the manufacturer, and as it exists in inventory in the FCC unit, has gradually increased over the last 10 to 15 years. The average particle (APS) size of FCC catalyst has increased from an average particle size of perhaps 70 microns in the period 1983-1985, to a particle size in the range of 85-90 microns (1988-1990). Some units now run with an APS well above 90 microns.
The increased particulate size, or perhaps the more durable catalyst formulations, have helped to reduce particulates emissions. Unfortunately, the coarser, tougher catalysts were also making the FCC units harder to control and to run. The catalyst just does not fluidize and flow from reactor to regenerator and so on.
The problem was severe in some units. Some improvement in fluidization was achieved by addition of fluidization aids, material of 20-40 micron size whose primary function is to help coarse FCC catalyst circulate in the unit. Catalyst vendors sell this material.
U.S. Pat. No. 5,393,412 taught a "triangular" approach to catalyst management, taking some of the circulating ECAT out of the unit and preferably subjecting to classifying, attrition, and magnetic separation.
The magnetic separation, attrition, and screening approach of '412 patent gave refiners a way to change ECAT physical properties, but required a significant amount of processing of catalyst in attrition and separation means, in addition to magnetic separation. The '412 patent recognized the difficulty caused by coarse materials (burping of the bed and poor oil contact) and the beneficial effect of some fine particles. It summarized the state of the FCC art by stating:
"further it has been established by experience over many years by many refiners, that a particle distribution most preferably in the 40-80 micron range, as mentioned above, gives best overall performance."
While the approach disclosed in '412 will help significantly, it requires a significant capital and operating expense. This can be justified when processing a heavy, metals laden feed, but can not be justified when processing a clean feed. Refiners frequently just need a way to improve fluidization, especially on short notice if a batch of makeup catalyst is unusually coarse or if some unit upset causes a significant loss of fines causing fluidization difficulties.
Another concern with the "triangle" approach is that this expensive equipment is used fairly aggressively on a small fraction of the catalyst inventory. It takes months to turn over the inventory of an FCC in a magnetic separation unit, so there is a tendency to somewhat overtreat the modest fraction passed each day through the external processing facility. While this is probably the best use of the capital and operating expense of the external magnetic separation/grinding/sieving unit, it is probably overkill for the fraction removed and treated. Thus the grinding operation probably takes off an excessive amount of catalyst structure. If ECAT were children playing in mud, the triangle approach is akin to giving them a thorough scrubbing one a month or so when they can be found, whereas more frequent and milder treatment would be better for the individuals being subjected to such treatment.
I wanted to be able to treat a larger fraction of the catalyst inventory than is practical in most magnetic separation operations, but do less to each catalyst particle. For optimum handling and use of catalyst, my philosophy was roughly, treat more catalyst, but do less to each particle per treatment.
I discovered a gentler way to treat more of the ECAT. This new approach allows refiners to create fines, in situ, and on demand, so that FCC catalyst will have better catalyst circulation properties. This approach can be used to reduce average FCC catalyst particle size and even increase conversion to a limited extent. In units running with modest amounts of metals in the feed, the gentle approach provided a measure of demetallization of the catalyst by preferentially removing metals. Most of the metals, except for mobile vanadium species, deposit on the exterior of the catalyst. Abraiding off just a thin metals laden shell produces a metals enriched fines fraction which can be removed from the unit with the flue gas or the slurry oil and a smaller more readily fluidizable and somewhat demetallized ECAT. The metals rich fines will be then recovered using the technology already in place for fines recovery.